The present invention relates to an automatic polishing apparatus for automatically polishing free curved surfaces such as the surfaces of molds.
The specification of U.S. Pat. No. 3,939,610 discloses an automatic polishing apparatus of self-guided constant pressure profiling type. This apparatus has a polishing tool carried by an arm which is adapted to be automatically swung to perform a profiling action in the Z-axis direction, i.e. in the depthwise direction. The control of the position of polishing tool on the surface to be ground as viewed in plan, i.e. in the plane defined by X and Y-axes, is achieved by making use of limit switches adjustably disposed along respective axes, in such a manner that the arm makes a constant-speed linear reciprocating movement the direction of which is changed as the limit switches are engaged by cams. Thus, the arm carrying the polishing tool is moved in accordance with the composite instruction which is obtained by composing the profiling instruction in Z-axis direction and the tabular movement instruction given by the limit switches.
This apparatus, however, can effectively be used only when the surface to be ground has a simple planar shape such as a rectangular shape, and cannot satisfactorily trace the complicated planar shape of the surface such as the surface of a plastic mold. This apparatus, therefore, cannot perform an efficient and good polishing and finishing of such a surface having a complicated planar shape.
To overcome above described problem, it is proposed to effect the control of tabular movement of the polishing tool in the plane defined by X and Y-axes, making use of a numerical control device. This method, however, requires strictly precise instructions for the positions of centers of arcs, points of intersections of locus and so forth, because the path of movement of the polishing tool is given as a combination of straight lines and arcs. Therefore, when the polishing tool is to be moved along a complicated path, the number of points of intersection are inevitably increased to require an impractically long time for the arithmetic operation or calculation of positions of these points. In addition, in the proposed control method employing an NC device, the actual polishing is performed after a tape check of an NC tape which is formed with these numerical data. The formation and check of the NC tape requires a considerably long time. This method, therefore, is quite unsuitable for the polishing of plastic molds which are usually produced one by one for each of different configurations of articles to be shaped by molding.